Abstract
We explore the interplay of electron-electron correlations and surface effects in the prototypical correlated insulating material, NiO. In particular, we compute the electronic structure, magnetic properties, and surface energies of the $(001)$ and $(110)$ surfaces of paramagnetic NiO using a fully charge self-consistent DFT+DMFT method. Our results reveal a complex interplay between electronic correlations and surface effects in NiO, with the electronic structure of the $(001)$ and $(110)$ NiO surfaces being significantly different from that in bulk NiO. We obtain a sizeable reduction of the band gap at the surface of NiO, which is most significant for the $(110)$ NiO surface. This suggests a higher catalytic activity of the $(110)$ NiO surface than that of the $(001)$ NiO one. Our results reveal a charge-transfer character of the $(001)$ and $(110)$ surfaces of NiO. Most notably, for the $(110)$ NiO surface we observe a remarkable electronic state characterized by an alternating charge-transfer and Mott-Hubbard character of the band gap in the surface and subsurface NiO layers, respectively. This novel form of electronic order stabilized by strong correlations is not driven by lattice reconstructions but of purely electronic origin. We notice the importance of orbital-differentiation of the Ni $e_g$ states to characterize the Mott-Hubbard insulating state of the $(001)$ and $(110)$ NiO surfaces. The unoccupied Ni $e_g$ surface states are seen to split from the lower edge of the conduction band to form strongly localized states in the fundamental gap of bulk NiO. Our results for the surface energies of the $(001)$ and $(110)$ NiO surfaces show that the $(001)$ facet of NiO has significantly lower energy. This implies that the relative stability of different surfaces, at least from a purely energetic point of view, does not depend on the presence or absence of magnetic order in NiO.
Highlights
Our results reveal a complex interplay between electronic correlations and surface effects in NiO, with the electronic structure of the (001) and (110) NiO surfaces being significantly different from that in bulk NiO
We begin with the electronic structure and equilibrium lattice volume calculations of bulk NiO in the paramagnetic state using density functional theory (DFT) + DMFT [51,52,89]
In the surface layer χ (τ ) is seen to be remarkably larger at all τ, implying higher localization of the surface eg states. This leads to a higher charge-transfer character of the (001) NiO surface, while for the bulk our results suggest a mixture of a Mott-Hubbard d-d and charge transfer character of the band gap [20,21,24]
Summary
The series of transition metal monoxides MnO, FeO, CoO, and NiO with an electronic configuration ranging from 3d5 to 3d8, respectively, has attracted much attention due to their diverse electronic and magnetic properties [1,2,3,4,5], allowing for a broad range of applications, e.g., in electronics and spintronics [6,7,8,9], energy storage [10,11,12,13], and heterogeneous catalysis [14,15,16,17] At low temperature, these compounds exhibit a correlated Mott-Hubbard or charge-transfer insulating behavior with a large band gap of ∼2-4 eV, associated with a strong localization of the 3d electrons [18,19,20,21,22]. We study the electronic structure, magnetic properties, and surface energies of the (001) and (110) NiO with particular attention given to the effect of structural confinement and its influence on the strength of electronic correlations in the PM (001) and (110) NiO
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